Blizzard has posted an announcement that there was a security breach into their internal network within the past week. Blizzard reacted quickly to secure all information, and does not believe financial information such as credit cards, billing addresses, or real names were compromised. They describe what information was illegally accessed in the paragraph below:
Some data was illegally accessed, including a list of email addresses for global Battle.net users, outside of China. For players on North American servers (which generally includes players from North America, Latin America, Australia, New Zealand, and Southeast Asia) the answer to the personal security question, and information relating to Mobile and Dial-In Authenticators were also accessed. Based on what we currently know, this information alone is NOT enough for anyone to gain access to Battle.net accounts.
Also, scrambled versions of Battle.net users' passwords have been stolen, and as a precaution, Blizzard advises everyone to change their passwords, secret questions, and the answers to those questions. For the full Blizzard post written by Mike Morhaime, click here.
Battle.net - Security Update